MCR Geothermal Corporation pioneered successful exploratory drilling the Bottle Rock area of the Geysers Steam Field in 1976. The wellfield is characterized by a deep reservoir with varied flowrates, temperatures, pressures, and steam chemistries being quite acceptable. More detailed reservoir engineering tests will follow as production commences. Subsurface geology is comprised of a deep main graywacke rock(s) with overlying cap, condensation, and highly altered r e servoir zones. Liquid reserves for steam production probably exist in the condensation zone fractures and alteration pores(?), and by physical adsorption in micropores along fractures u r faces, since interstitial porosities and the presence of a basal boiling brine have been ruled out. The steam reservoir evolved from ancestre.1 liquid-dominated hydrothermal systems and possesses an extremely viable fracture system comprised of tectonic rubble breccias. Essentially, the steam reservoir(:) are huge tectonic breccias superimposed ca (mimicing) and now overlapping huge a n c e s t r a l hydrothermal breccias. While earlier reservoir models utilized a boilingbrine and/or Raleigh-plume dispersion mode of existance and operation, geological data now suggests that the r e servoir may be a system of v e r t i c l e steam-filled fractures with a separate condensation zone held somewhat immobile along the top and flank boundaries of the fracture networks. Local extensional tectonics have and continue to play a very important role in the formation and continuance of both the ancestral liquid and active vapor systems in the steam field. Accordingly so, drilling programs should be designed with tectonics in mind so as to maximize well deliver a b i l i - ties. The Francisco lease will supply steam to power a 55 MW powerplant presently under construction by the California Department of Water Resources.

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